Justification:
Using a generation length of between 4.5 and 6 years, we estimated a decline in overall stock abundance of 31–38% over 14 years and 18 years, respectively. Global declines were calculated as a weighted average of the declines for each stock using maximum historical catch as a proxy for the stock's contribution to the global population. Based on the available data, this species is listed as Vulnerable under criterion A2. This species is not considered to be well managed in any part of its range. It is important to note that the Pacific stock comprises over half of the global population for this species, and that declines in the Pacific were estimated using catch data as no recent standardized indices were available. It is likely that declines based on these data are conservative. Fishing effort in the Pacific and Indian Oceans is increasing as a result of longline fisheries operating at deeper depths and in the Atlantic due to expansion of artisanal fleets. There is urgent need for an updated stock assessment for the Pacific Ocean and a reassessment of this species may be warranted when newer information becomes available. Data reporting from the Indian Ocean is also poor. We recommend that appropriate fishery statistics be compiled and analysed to accurately assess the condition of this species.

An epipelagic oceanic species, blue marlin is often found in wide open blue waters with surface temperatures between 22° and 31°C. It is the most tropical of the billfishes. Its latitudinal range changes seasonally, expanding northwards and southwards in the warmer months and contracting towards the equator in colder months.

In the Eastern Pacific it is found from California to the southwestern and central eastern Gulf of California to Peru, including all of the oceanic islands. In the Atlantic Ocean, adults are commonly found in the tropics within the 24°C isotherm.

In both the eastern tropical Pacific and the eastern tropical Atlantic, Blue Marlin concentrate in shallower waters than in the western part of both oceans due to hypoxia-based habitat compression over oxygen minimum zones in the eastern tropical seas (Prince et al. 2010).

Atlantic OceanThe most recent stock assessment for Blue Marlin (ICCAT 2007) used a Bayesian Surplus Production model to estimate biomass from 1990 through 2006. The remaining data series (1956–1989) are based on biomass estimates from the previous stock assessment (ICCAT 2002). Population declines were examined using a generation length estimated between 4.5 and 6 years. Over a three generation length period of 14 years, the decline was 60% and over a three generation length period of 18 years, the decline was 64%. These declines were calculated using a linear regression over each of the time periods examined. Population reduction for Atlantic blue marlin was therefore estimated to be between 60% and 64%. For all of the models for which the stock productivity was constrained to be near the productivity estimated by the last full assessment, the current biomass was estimated to be at or below biomass at maximum sustainable yield (BMSY) and current fishing mortality rates were above FMSY. The stock is therefore not considered to be well-managed.

Pacific OceanThe best knowledge currently available indicates that Blue Marlin constitutes a single world-wide species (Buonaccorsi et al. 1999, 2001, Collette et al. 2006), and that there is a single stock of Blue Marlin in the Pacific Ocean (Hinton 2001). No recent estimates of biomass or catch per unit effort (CPUE) were available. Biomass estimates from the last stock assessment for Pacific Blue Marlin (Kleiber et al. 2003) were available only until 1997. We therefore used estimated catch data from the Inter-American Tropical Tuna Commission (IATTC) (Hinton, unpublished data 2011), which were the only available recent data. We assume that effort has remained stable or has increased over the time period examined. Based on catch data and a generation length of 4.5–6 years, we estimated declines of 5% over 14 years (1996–2009) and 19% over 18 years (1992–2009) using linear regression. We consider that these estimates of decline (based on catches) are likely conservative. This stock is not considered to be well-managed and there is urgent need for a newer stock assessment to evaluate population trends.

Indian OceanNominal yearly CPUE of Japanese longliners in Northwest Australia has declined 58.9% over a 14 year period (1993–2007) and 70.6% over a 18 year period (1989–2007; Figure 40, IOTC 2009). The CPUE decline for Japanese longliners in the Seychelles was 79.5% over 14 years and 56.9% over 18 years (Figure 40, IOTC 2009). For each time period considered, an average of the declines in Northwest Australia and the Seychelles was used to characterize the population in the Indian Ocean. The decline in the Indian Ocean ranged from 63–69% depending on the time period considered. It is important to note that these data are limited and catch data from other industrial fisheries such as longliners of Indonesia and Philippines are not available. The stock is not considered to be well managed and more information is needed to understand population declines for blue marlin in the Indian Ocean.

This epipelagic and oceanic species is mostly confined to the waters on the warmer side of the 24°C surface isotherm and known to undergo seasonal north-south migrations. It is found to 1,000 m depth but spends the highest percentage of its time at shallower depths, and is not usually seen close to land masses or islands, unless there is a deep drop-off of the shelf. This species can dive as deep as 1,000 m, but it remains mostly within the upper 40 m. It is believed to form small-scale schools of at most 10 individuals. Larger fish tend to swim solitarily (Nakamura 1985) but smaller ones form aggregations.

It feeds on squids, tuna-like fishes, crustaceans, and cephalopods (Nakamura 1985). Spawning probably takes place year-round in equatorial waters to 10°N/S and during summer periods in both hemispheres to 30°N/S, in both the Indian and Pacific oceans (Kailola et al. 1993). In the southern hemisphere, concentrations of spawning fish probably occur around French Polynesia (Howard and Ueyanagi 1965). In Brazil, spawning occurs February to March from 20–23°S, primarily in the Abrolhos Archipelago (Amorim et al. 1998). Most of the individuals captured at this location have been juveniles (Amorim pers. comm. 2010).

Maximum time at large recorded is 11 years (Ortiz et al. 2003). Maximum age is estimated to be at least 20 years (Wilson et al. 1991). Maximum age is estimated in the Pacific as 27 years (females) and 18 years (males) (Hill et al. 1989). Age estimation in marlins is problematic and longevity information from the Pacific has also been applied to the Atlantic. Age at maturity is estimated to be two years (Prince et al. 1991, Torres-Silva et al. 2006). Using longevity estimates of 20 years and 27 years, and age of maturity of two years, the generation length was estimated to be between 4.5–6 years. The generation length is calculated as: age of first reproduction + z * (longevity - age of first maturity), where z is 0.15 (Collette et al. 2011).

The all-tackle game fish record is of a 636-kg fish caught off Vitoria, Brazil in 1992 (IGFA 2011).

Blue Marlin occurs as bycatch, primarily in longline fisheries for tuna. There is also a directed recreational catch for this species in many areas. The flesh is of good quality and is marketed frozen and prepared as sashimi and but most of it is marketed frozen or for sausages in Japan (Nakamura 1985).

In the Pacific, Blue Marlin are an incidental catch of longline fisheries, bycatch in swordfish fisheries, and an important resource for big game recreational fishing. More than 73% of reported landings are incidental to large offshore longline fisheries, and other major fisheries are the directed recreational fisheries of the USA and other countries (Restrepo et al. 2003). Protections efforts for blue marlin have continued to decrease in recent years, as deeper longline gear is introduced (Uozumi 1999).

In the Atlantic new fleets have harvested large catches of Blue Marlin, including artisanal fish aggregating devices (FAD) fisheries in the eastern Caribbean islands and a new artisanal fleet of small longliners operating off Brazil between 18°S and 30°S. This species is primarily taken as bycatch by longline fisheries, but also by purse seines, by some artisanal gears which are the only fisheries targeting marlins and also by various sport fisheries located on both sides of the Atlantic. The increasing use of anchored FADs by various artisanal and sport fisheries is causing greater vulnerability of these stocks (STECF 2009). For example, over the last fifteen years, Antillean artisanal fleets have increased the use of Moored Fish Aggregating Devices (MFADs) to capture pelagic fish. Catches of Blue Marlin caught around MFADs are known to be significant but reports on these catches made to the International Commission for the Conservation of Atlantic Tunas (ICCAT) are very incomplete (ICCAT 2006).

This is a highly migratory species, listed under Annex I of the 1982 Convention on the Law of the Sea (FAO Fisheries Department 1994).

In some areas, long-lining is restricted to protect fish stocks for sport fishing. Size limitations, encouragement of catch-and-release sport fishing, and recommendations for using circle hooks instead of J-hooks are measures designed to increase survival in catch-and-release sport fishing (Pine et al. 2008, Serafy et al. 2009). Marlin species are a special case because bycatch in the longline fisheries concentrating primarily on tunas causes the majority of fishery mortality (>90%) for marlin (Kitchell et al. 2004). In the Pacific, marlin are most frequently captured on the shallow hooks of a longline set (those close to the floats), and removing less than 15% of the hook sets adjacent to floats would decrease marlin catch by as much as 50% (Kitchell et al. 2004).

For Blue Marlin in the Atlantic, the ICCAT Standing Committee on Research and Statistics (ICCAT–SCRS) in 2008 asked the Commission, at a minimum, to continue the management measures already in place because marlins have not yet recovered. The Commission should take steps to assure that the reliability of the recent fishery information improves in order to provide a basis for verifying possible future rebuilding of the stocks. Improvements are needed in the monitoring of the fate and number of dead and live releases, with verification from scientific observer programs; verification of current and historical landings from some artisanal and industrial fleets; and complete and updated relative abundance indices from CPUE data for the major fleets. Should the Commission wish to increase the likelihood of success of the current management measures of the marlin rebuilding plan, further reduction in mortality would be needed, including: implementing plans to improve compliance of current regulations; encouraging the use of alternative gear configurations, including certain types of circle hooks, hook/bait combinations etc., in fisheries where its use has been shown to be beneficial; and broader application of time/area catch restrictions.

Given the recent importance of the catch from artisanal fisheries, and to increase the likelihood of recovery of marlin stocks, the Commission should consider regulations that control or reduce the fishing mortality generated by these fisheries. The Commission should encourage continued research on development of methods to incorporate this information into stock assessments in order to provide a basis for increasing the certainty with which management advice can be provided.

The Scientific, Technical and Economic Committee for Fisheries (STECF) stresses the need for correct identification and reporting of billfish species in all fisheries. Furthermore, STECF notes that the 2007 ICCAT–SCRS report indicated the potential for the stocks of blue marlin and white marlin to recover to the BMSY level. However, recent increases in catches of Blue Marlin by artisanal fisheries on both sides of the Atlantic may compromise the effectiveness of the ICCAT plan (STECF 2009).

Recent analyses suggest that the recovery of blue marlin stock might proceed faster than would have been estimated at the 2000 assessment, provided catches remain at the level estimated for 2004. Some signs of stabilization in the abundance trend are apparent in the most recent catch per unit of effort data of Blue Marlin (2000–2004) Recommendations [Rec. 00-13], [Rec. 01-10] and finally [Rec. 02-13] placed additional catch restrictions for Blue Marlin and White Marlin. The first established that the annual amount of Blue Marlin that can be harvested by pelagic longline and purse seine vessels and retained for landing must be no more than 33% for White Marlin and 50% for Blue Marlin of the 1996 or 1999 landing levels, whichever is greater. That recommendation established that all Blue Marlin and White Marlin brought to pelagic longline and purse seine vessels alive shall be released in a manner that maximizes their survival. The provision of this paragraph does not apply to marlins that are dead when brought along the side of the vessel and that are not sold or entered into commerce. Catches of both species have declined since 1996–99, the period selected as the reference period by the recommendations. Since 2002, the year of implementation of the last of these two recommendations, the catch of Blue Marlin has been below the 50% value recommended by the Commission. This analysis represents only longline caught marlin even though the recommendations referred to the combined catch of pelagic longline and purse seine because the catch estimates of billfish by-catch from purse seine vessels are more uncertain than those from longline. More countries have started reporting data on live releases in 2006. Additionally, more information has come about, for some fleets, on the potential for using gear modifications to reduce the bycatch and increase the survival of marlins (ICCAT 2007).

Catches of Blue Marlin in the Indian Ocean are very poorly reported and there is need of better data to evaluate the condition of the Indian Ocean stock.